You're assuming that it's a choice people want to make. If you talk to a few gig economy workers it becomes pretty clear that many of them would much prefer a 9-5 job with decent pay and benefits, but are unable to find such jobs.
Many of them are people seeking a little extra money, a way to fund a new car, something to hold them over between jobs, a way to pick their own schedule... I have had a lot of conversations with a lot of workers in the gig economy and never have heard "I wish this was my full time job". Maybe that is the random set of people I've interacted with, maybe there is a bias for what people want to talk about.
What is absolutely clear though, is that a sizable amount of people participating in the gig economy are doing so because they want to take advantage of how it works, not despite how it works.
Many of them had other jobs, many of them clearly wouldn't want a full time job doing the gig.